As one of technologies that realize the large capacity of an optical transmission system, wavelength division multiplexing (WDM) has been put into practice. In the WDM, transmission of a multi-channel optical signal (that is, WDM signal) is performed in which a plurality of optical signals having different wavelengths (or optical frequencies) is multiplexed.
In the optical transmission system that performs transmission of a WDM signal, an optical add drop multiplexer (OADM) is provided in each node. The OADM may drop a specified optical signal from the WDM signal. The optical signal that is dropped from the WDM signal is guided, for example, to a client device. In addition, the OADM may add an optical signal that has been received from the client device or a further network, to the WDM signal.
The above-described add/drop function of the OADM is realized, for example, using a wavelength selective switch (WSS). The WSS may select an optical signal having a desired wavelength, for example, using an array waveguide grating, a micromachine, a liquid crystal, or the like.
In Japanese Laid-open Patent Publication No. 2012-119925, a reconfigurable OADM (ROADM) device and an optical add drop method are discussed.
When the wavelength interval of the WDM signal is reduced in the optical transmission system, the transmission capacity is further increased, and the flexibility of the network may be improved. However, in an existing WSS, it is difficult to realize sharp light-transmission characteristics, and there is a limitation of the wavelength intervals of a WDM signal.